1. Field of the Invention
The present invention relates to a method of testing a phase shift mask for photolithography used in manufacturing LSIs (Large Scale Integration) and a testing apparatus used in this method. Particularly, it relates to accurate measurement of the amount of phase shift and transmittance.
2. Description of the Background Art
Photographic techniques are well known for forming a desired LSI pattern on a semiconductor wafer using an optical stepper. An optical stepper is an exposure apparatus that reduces an enlarged pattern on a photomask onto a wafer for projecting the same in a step-and-repeat manner.
FIG. 22 schematically shows the main components of such an optical stepper. Referring to FIG. 22, an optical stepper includes a light source 31, a condenser lens 32, a demagnification lens 33, and an X-Y stage 34. Light emitted from light source 31 is directed onto a photomask 1 by condenser lens 32. Light transmitted through photomask 1 is projected onto a wafer 35 set on X-Y stage 34 by demagnification lens 33.
FIG. 23 shows an example of a conventional photomask used in an optical stepper. Referring to FIG. 23(A), a photomask 1A includes a transparent substrate 1a of flat glass or the like. A light shielding pattern such as of MoSi is formed on transparent substrate 1a. The material of light shielding pattern 2 is selected so that light used in photolithography can be blocked sufficiently. In photolithography, a g-line (wavelength .lambda.=0.436 .mu.m) or an i-line (.lambda.=0.365 .mu.m) which are emissions of a mercury lamp, or excimer laser can be used. Photomask 1A passes light only through light transmitting portions 1b.
FIG. 23(B) shows an electric field distribution of light just passing through photomask 1A. It can be seen that the electric field distribution exhibits a pattern of light transmitting portions 1b in fidelity right after light passes through photomask 1A.
If the distance between light transmitting portions 1b is reduced to improve the integration density of a LSI, the light amplitude on a semiconductor wafer 35 is as shown in FIG. 23(C). Because light transmitted through photomask 1A slightly bends laterally as a function of advance due to diffraction, light passing through two adjacent light transmitting portions 1b will interfere with each other. When two adjacent light transmitting portions 1b come closer to each other than a certain limit, the light intensity distribution can no longer define the two adjacent light transmitting portions 1b on a semiconductor wafer as shown in FIG. 23(D). This means that the pattern on photomask 1A is not resolvable. It is to be noted that the horizontal access in (C) and (D) in FIG. 23 is increased in scale to clarify the corresponding relationship with photomask 1A.
As shown in FIG. 23(E), the pattern on photomask 1A will not be produced in fidelity on wafer 35.
The resolution limit R of an optical stepper contributing to miniaturization of a LSI pattern is expressed by the following equation (1). EQU R=k.multidot..lambda./NA (1)
The resolution of an optical stepper increases with a smaller value of resolution limit R. k is a constant dependent upon the photoresist process, and can be set as low as approximately 0.5. .lambda. represents the wavelength of light used for exposure, and NA represents the numerical aperture of the lens.
It can be understood from the above equation (1) that the resolution limit R is reduced (that is to say, the resolution is increased) by reducing constant k and wavelength .lambda., and increasing the numerical aperture NA of the lens. Because the maximum value of numerical aperture NA can be set to approximately 0.5 according to the current stage of art, the value of resolution limit R can be reduced to approximately 0.4 .mu.m using an i line (.lambda.=0.365 .mu.m).
In order to obtain a lower value of resolution limit R, numerical aperture NA should be further increased or light having a shorter wavelength .lambda. should be used. However, the design of a light source and a lens is technically difficult for this purpose. A depth of focus 6, which is represented by the following equation (2), can be set to a lower value by reducing wavelength .lambda. and increasing numerical aperture NA to reduce resolution limit R. EQU .delta.=.lambda./{2(NA).sup.2 } (2)
Thus, there was a problem that the resolution cannot be improved integrally. The usage of a phase shift mask is known in the prior art to avoid such a problem.
FIG. 24 shows a phase shift mask disclosed in Japanese Patent Laying-Open No. 58-173744, for example. The phase shift mask of FIG. 24(A) is similar to the photomask of FIG. 23(A) except for the provision of a phase shifting portion 3 formed of a transparent material such as SiO.sub.2. A light transmitting portion 1b and a phase shifting portion 3 are disposed alternately. The phase of light passing through phase shifting portion 3 is shifted by 180.degree. with respect to light passing through light transmitting portion 1b.
FIG. 24(B) shows the intensity distribution of an electric field by light right after passage through phase shift mask 1. The intensity distribution at the minus side shows that the phase is inverted with respect to the intensity distribution of the plus side.
FIG. 24(C) shows the amplitude of light on a wafer where light having the electric field distribution of FIG. 24(B) is projected. Light transmitted through the mask of FIG. 24(A) slightly expands laterally by diffraction, as in the case where light is transmitted through the mask of FIG. 23(A). However, light transmitted through the adjacent light transmitting portion 1b and phase shifting portion 3 have a relation of inverted phase with respect to each other. Therefore, the interference therebetween serves to cancel the light intensity. The light intensity pattern on the wafer can be identified even if the distance between light transmitting portion 1b and phase shifting portion 3 is reduced as shown in FIG. 24(D). The phase shifting mask 1B of FIG. 24 can have the minimum resolvable pattern width reduced to approximately a half in comparison with that of photomask 1A of FIG. 23 according to experiments.
As a result, the pattern on phase shift mask 1B is produced in fidelity on wafer 35 as shown in FIG. 24(E).
In phase shift mask 1B of FIG. 24, light transmitting portion 1b and phase shifting portion 3 must be disposed alternately. Although the phase shift mask of FIG. 24 can easily be adapted for simple and periodic patterns such as line patterns and space patterns, it is not suitable for complex patterns. It is difficult to improve the resolution in all regions of patterns having an arbitrary configuration.
A phase shift mask 1C applicable to arbitrary patterns and that can be manufactured easily is disclosed in IEDM (1989, pp. 57-60) by Nitayama et al. to solve the above problem. This phase shift mask can be formed by self-alignment. Referring to FIG. 25(A), a phase shifter 3 has a width larger than that of a light shield pattern 2. Therefore, light passing through the proximity of the edge of light shielding pattern 2 has its phase inverted by phase shifter 3, as shown in FIG. 25B. Therefore, the amplitude of light on wafer 35 is as shown in FIG. 25(C). As a result, phase shift mask 1C of FIG. 25(A) can realize a light intensity distribution on wafer 35 that closely corresponds to light transmitting portions 1b as shown in FIG. 25(D) whatever the pattern shape may be. Therefore, the pattern of phase shift mask 1c is produced in fidelity on wafer 35 as shown in FIG. 25(E).
An attenuation type phase shift mask is disclosed in JJAP Series 5 Proc. of 1991 Intern. Micro Process Conference Opp. 3-9 and in Japanese Patent Laying-Open No. 4-136854 as a phase shift mask applicable for arbitrary patterns and that can be easily implemented from the standpoint of the manufacturing process. FIG. 26 shows such an attenuation type phase shift mask. This attenuation type phase shift mask 1D includes a phase shifting portion of a double layered structure of a chromium layer 35 with a light transmittance of 5-40% and a shifter layer 3 providing a phase difference of 180.degree. to the transmitting light, as shown in FIG. 26(A).
Light transmitting this phase shifter portion has its phase inverted and results in a light transmittance of 5-40%, as shown in FIG. 26(B). Therefore, the amplitude of light on wafer 35 is as shown in FIG. 26(C). Because the phase is inverted at the edge of the pattern, light intensity becomes 0 at the edge of the exposure pattern to allow high resolution. Therefore, attenuation type phase shift mask 1D of FIG. 26(A) allows a light intensity distribution accurately corresponding to light transmitting portion 1b to be realized on the wafer as shown in FIG. 26(D) regardless of the configuration of the pattern. Thus, the pattern of attenuation type phase shift mask 1D is produced in fidelity on wafer 35 as shown in FIG. 26(E).
Phase shift masks 1B, 1C and 1D are designed so that the phase difference between light transmitted through light transmitting portion 1b and light transmitted through phase shifting portion 3 is 180.degree.. The effect of improving the resolution is degraded as the actual phase difference is offset from 180.degree.. Therefore, it is necessary to test the actual amount of phase shift in the formation step of a phase shift mask. Conventional testing of the amount of phase shift in a shift mask includes the steps of measuring the refractive index and thickness of a phase shifting portion 3 and calculating the amount from the measured results.
When an ellipsometer (polarization diffracting device) is used for measurement, the refractive index cannot be measured because reflection from the substrate boundary is too low due to the formation of a phase shifting portion 3 on a transparent substrate 1a. In practice, the refractive index of the phase shifter formed on a substrate such as of silicon is measured. However, the produced transparent substrate of the phase shift mask and the silicon substrate differ in material. Therefore, the quality of the film of the phase shifter formed on these substrates also differ, giving rise to the problem that the refractive index is different.
When a needle type profilometer is used to measure the thickness of a phase shifter, those having the practical pattern interval of approximately 1 .mu.m-2 .mu.m cannot be measured due to the limitation of the size of the needle tip. In determining the thickness of a phase shifter with such a needle type profilometer, a coarse pattern for the purpose of measurement is formed, and the thickness of the phase shifter included in that pattern is measured. However, the thickness of the phase shifter on the actual pattern cannot be identified if the thickness of the shifter included in the pattern of the actual phase shift mask differs from the thickness of the phase shifter included in the coarse pattern formed for measurement.
Takahiro Ode discloses a phase shift amount measurement device of optical heterodyne method in pp. 22-29 in Proceedings of 11th Meeting of Japan Society for Laser Microscopy (1993). This phase shift amount measurement device uses an HeNe laser as a light source having a wavelength of 0.633 .mu.m. An HeNe laser having such a long wavelength is not used in the photolithographic step using a phase shift mask. The refractive index of a phase shifter measured with an HeNe laser differs from the refractive index of ultraviolet light of, for example, 0.365 .mu.m in wavelength used in photolithography. Because ultraviolet light cannot be used as a light source in an optical heterodyne phase difference measurement device, there was a problem that the phase difference in a phase shift mask cannot be obtained accurately for ultraviolet light that is used in photolithography.
In the case of a signal processing system using a shearing system Mach-Zehnder interferometer of a shearing type for visible light as shown in JENA REVIEW 10 (1965) 99-105, "Interferenzeinrichtung fur Durch lichtmicroskopie" Beyer, H. and Shoppe, G. and JENA REVIEW 16 (1971) 82-88 Special Fair Issue "Auflicht-Interferenzmicroscop EPIVAL interphako" Beyer, H., the refractive index of the phase shift pattern material of the phase shift mask differs from the refractive index at the wavelength of 365 nm, for example, which is actually use for exposure since the wavelength of light used for measurement of the amount of phase shift is within the visible light range.
Thus, there was the problem that the amount of phase shift of the phase shift pattern material could not be obtained with a wavelength that is actually used for exposure. Furthermore, because the above-described signal processing system uses a homodyne system, a correct amount of phase shift cannot be obtained if only a weak interference beam is obtained.
In Japanese Patent Laying-Open No. 3-181805, a shifter film thickness measurement apparatus is disclosed of a phase shift mask having a sample disposed in the optical path of the Mach-Zehnder interferometer. This shifter film thickness measurement apparatus will be described schematically with reference to FIG. 27. Referring to FIG. 27, light emitted from a white light source 48 is rendered into parallel rays by a lens 56 to be diverged into an optical path 46A and an optical path 46B by a half mirror 46a. Optical path 46A passing through a shifting portion 56 of a phase shift mask 52 is restored into parallel rays by a lens 56b after transmitting through a shifting portion 59 as a spot light from lens 56a. Optical path 46A with the parallel rays are combined with optical path 46B by a half mirror 46c via a wedge-like glass 49 for course adjustment and a half mirror 46b. Wedge-like glass 49 is provided for course adjustment when optical path 46A deviates from optical path 46B in the optical path length.
The diverged light is incident into a detector 45a by half mirror 46b. By this detector 45a that detects light intensity, light shielding patterns and detective portions of light transmitting portions can be identified. The light diverged by half mirror 46a and reflected by a mirror 47 passes through a dummy glass substrate 54 and a wedge-like glass 50 for fine adjustment, so that optical path 46B is combined with optical path 46A by half mirror 46c. Dummy glass substrate 54 serves to approximate the optical path length of optical path 46B to that of optical path 46A, and has a thickness identical to that of the glass substrate of phase shift mask 52. Wedge-like glass 50 is controlled to be fine-moved by a pulse motor 51.
In a shifter film thickness measurement apparatus of the above-described structure, pulse motor 51, light interference signal detector 45a, and an X-Y stage 53 that moves light intensity detector 45b and phase shift mask 52 vertically and horizontally are controlled by a control device 44, whereby the presence and film thickness of shifting portion 59 is measured. The pattern configuration of the phase shift mask is also detected.
In the above-described shifter film thickness measurement apparatus, interference is low since white light source 48 is used. An interference waveform cannot be obtained when there is difference in the optical path length within the interference portion of the interferometer. It is therefore necessary to prepare a dummy glass substrate 54 having a thickness identical to that of the sample. There is also a problem that the phase at a wavelength that is actually used for wafer exposure cannot be measured accurately since white light source 48 is used.
A difference in length between the two optical paths is generated due to tremble in the air inside the interferometer or due to the vibration of the components forming the interferometer, resulting in a swing in the intensity of the interference signal. It is desirable to suppress the deviation in the difference between the two optical path length to less than approximately .lambda./1000. To satisfy this limit, a structure that blocks the exterior environment and an anti-vibration mechanism are required. However, even if the air is completely sealed, insertion and removal of the phase shift mask and the dummy glass substrates and movement of X-Y stage 53 for pattern positioning at the time of measurement cannot be avoided, resulting in tremble in the air inside the interferometer. Thus, it is difficult to suppress the internal turbulence to less than .lambda./1000.
In using an attenuation type phase shift mask, the phase shift pattern has a light transmittance selected from the range of approximately 5-40%. However, when this light transmittance is offset from the designed value, for example when the light transmittance is too low, the role as a phase shift mask is deteriorated to approximate the characteristics of a photomask having the general light shielding pattern. If the light transmittance is too high, light will leak from regions which actually should shield light, resulting in a fog phenomenon. It is therefore necessary to set an appropriate value of the light transmittance for the phase shift pattern.
As shown in "Problems Regarding a Micro Pattern Densitometer" in Transactions of the Society of Illumination 1942, light transmittance measurement of fine patterns involved a phenomenon called Schwarzschild-Villiger effect. More specifically, there is a great error in the measured value due to stray light caused by reflection of light between the surface of the objective lens and the phase shift mask, and by reflected light within the objective lens. This phenomenon will be described hereinafter with reference to FIG. 28.
Referring to FIG. 28, an objective lens 4 and a light source 39 are disposed beneath an attenuation type phase shift mask 1. An optical system 4, a barrier wall 37 with a small opening, and a light receiver 36 are provided above attenuation type phase shift mask 1.
The light beam indicated by a solid line in the drawing implies light transmitting through a normal optical path. The light beam indicated by dashed lines implies stray light reflected by objective lens 4 and the surface of attenuation type phase shift mask 1 to enter light receiver 36.
In measuring the amount of phase shift in a micro phase shift pattern, there are cases the measure value differs from the actual amount of phase shift due to wavefront aberration by defocus with respect to the phase shift mask. This state is shown in FIG. 29. It is appreciated from FIG. 29 that wavefront 40 of the diffractive light from the peripheral patterns causes gradual increase in aberration 43 as being offset from focus 41 with respect to wavefront 44 of zero degree of the pattern to be measured.